GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Insulating stone and mortar

musictech99 | Posted in GBA Pro Help on

I read the excellent articles on insulating down in the basement. Very helpful as we prepare to finish the space. Our foundation is stone and mortar and the area we are finishing is all above grade and dry. Without your help we would likely have insulated the walls with fiberglass batts. The ceiling (underside of the first floor) was already insulated with fiberglass including stuffing the cavity at the rim joist. I now know I have to go with foam and make this airtight. Here’s my question:

Why can’t I use rigid foam for the walls? Even though it won’t be making consistent contact with the walls, I can make the foam continuous from the rim joist foam all the way down to the basement slab. Would that do? I guess I’m afraid that a spray foam contractor is going to be a fortune…

Somers, NY

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-2310254 | | #1

    Michael. Are you living in an older home? I ask because you say the foundation is "stone and mortar." That makes we wonder about how the home is framed. You might want to check out this discussion to see if you have a similar situation:

  2. musictech99 | | #2

    Not quite turn-of-the-century as in the story you linked to. Built around 1936. We definitely have a rim joist. What is a bit unusual is that there doesn't seem to be a sill plate; joists are in direct contact with the stone foundation wall. My interest is in being able to insulate (and frame) inside the stone foundation wall to create a finished basement. Hoping to avoid hiring a spray foam company.

  3. user-2310254 | | #3

    Here is a post from someone with a similar foundation and issue.

    The DIY foaming approach might be worth checking out. Is there any evidence that the joists ends have started to rot since they are sitting on stone?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |